The FCC gasoline obtained by means of the fluidized catalytic cracking of heavy hydrocarbon oil or vacuum gas oil contains sulfur compounds. Recently, in view of the environmental concerns such as prevention of air pollution or the like, it is required that a sulfur content in the FCC gasoline should be reduced because catalyst for removing NOx contained in exhaust gas from vehicles rapidly decreases its activity due to the effect by the sulfur content. An amount of sulfur contained in gasoline is regulated to be less than 50 ppm at the end of 2004 in Japan, and there have been proposed various methods for desulfurization of FCC gasoline in the FCC unit.
For instance, Japanese Patent No. 3545652 discloses a method for reducing sulfur contents in catalytic cracked oil fractions. This method includes a step of catalytic cracking feed oil fractions containing organic sulfur compounds in the presence of catalyst for reduction of sulfur contents at a high temperature and under the fluidized catalytic cracking conditions to produce a liquid product with the sulfur content substantially reduced. The commercially available catalyst for reduction of a sulfur content is porous molecular sieve, contains oxidized vanadium metal in an oxidized state of more-than-zero in inner pores of the molecular sieve crystalline structure, and the vanadium metal is present as a cation species produced by ion exchange in the pores of the sieve crystalline structure.
However, because the commercially available catalyst for reduction of sulfur contents contains oxidized vanadium metal in an oxidized state of more-than-zero in inner pores of the molecular sieve structure and the vanadium structure is present as a cation species produced by ion exchange in the pores of the sieve structure, the vanadium metal breaks a crystalline structure of the molecular sieve to disadvantageously lower the capability for catalytic cracking oil feed fractions.
Japanese Patent Laid-Open No. 2003-27065 discloses a desulfurization method for catalytic cracked gasoline with catalyst for desulfurization by means of catalytic cracking, and in this method, feed oil is catalytically cracked in a fluidized catalytic cracking unit or a heavy oil fluidized catalytic cracking unit with catalyst homogeneously carrying at least one type of metal selected from a group consisting of vanadium, zinc, nickel, iron, and cobalt on an inorganic porous carrier. This patent document also describes that it is preferable to use vanadium or zinc from the view point of desulfurization of the produced gasoline fractions.
However, the catalyst homogeneously carrying vanadium on the inorganic porous carrier has low affinity with sulfur compounds, so that the activity for desulfurization is low, and the improvement has been desired. Furthermore, if the inorganic porous carrier is fluidized catalytic cracking catalyst (sometimes referred to as FCC catalyst hereinafter) containing Y-zeolite, the catalyst has the effect of removing a sulfur content in gasoline fractions when heavy hydrocarbon oil is catalytically cracked, but because the Y-zeolite is destroyed by vanadium, the cracking activity drops, so that mounts of generated hydrogen and coke disadvantageously increase.